Sizing a mini split system

[SloppyDogDrool]

I have a 30x37x12 workshop. Pole barn construction. Walls are studded with Hardiplank siding. 2 garage doors, 10x16 and 10x10 insulated with 2" blue foam board. Garage doors are full sun about 2/3rd of the day. Back of building is shaded. The doors really cook! My intentions are R13 in the walls and R30 in the ceiling paper backed but I didn't plan on sheet rock for either.

So my question is, how do I accurately size a mini split system? I used several online calculators. One tells me I need 23K BTU and the next tells me 44K BTU and others anywhere in between.

From what I've been able to read, considering the size of the space I'm thinking I might need a dual zone unit.

So does anyone here have an opinion from experience or profession?

 

[rickairmedic]

I figure 500 SF to a ton for standard residential sizing . A ton is 12,000 BTU's with the insulation you will be in that area . I personally would figure less square footage per ton with your setup. I would also not leave the craft backed insulation bare in a working garage ( it is highly flamable ).

Rick

 

[jkeyser14]

I figure 500 SF to a ton for standard residential sizing . A ton is 12,000 BTU's with the insulation you will be in that area . I personally would figure less square footage per ton with your setup. I would also not leave the craft backed insulation bare in a working garage ( it is highly flamable ).

Rick

Guessing isn't a good idea, it will only waste his money. He should look up some manual J calculators online that will give him a proper tonnage. There are some that offer free trial downloads such as this one:

http://www.hvaccomputer.com/

 

[MW1975]

I would agree with Rick I also figure 500 sq ft per 12000 BTU'S for residemtial. Last year I installed a 13,000 BTU Fujitsu mini split 110v in a open office area about 600 sq ft they are verry happy with it. Hope this helps.

 

[Steves32]

Guessing isn't the right answer. Sizing based on square footage is a wild guess. Sizing a mini-split is no different than any other system.
You will need to do a Manual J calc on the garage.
Things to consider-
Orientation to north
Glazing
Flooring (yours is concrete)
Height
R values of walls & ceiling
Infiltration (garages are typically high)
Design temps for your area
Types of doors

The average price of a Manual J around here is around $250.00
If you choose a contractor to supply & install it, a Manual J will be included as part of the job (we do).

 

[Steves32]

Here's a pretty good one. Limited free trial, then you pay. I don't remember how many times you can open it before it locks you out asking for product code.

http://www.hvaccalc.com/hvac/newfeatures.asp

 

[rickairmedic]

Steve I actually do Manual J's as well and in most cases I have found the average 30-40 year old home in my area works out to 500 SF to a Ton. I would say in the OP's case being a garage with decent insulation he would need to be in the 400-500 SF per ton area .

Rick

 

[SloppyDogDrool]

Thanks guys. The few contractors I've spoken with did not speak highly of mini splits. I'd say I called 5 and maybe 3 returned my call and so far none have shown with one supposed to come tomorrow. There is so much work in our area getting someone for new work is difficult. Especially for a garage workshop.

The few I spoke with did not speak well of the mini split and prefer the conventional HVAC heat pumps and tell me they are cheaper. I'll keep regular updates here as I get closer. Realistic install date is probably April or May.

 

[sneezer41]

Depends if you want cheap or good

They make more profit on conventional

Find new installers

 

[Steves32]

Thanks guys. The few contractors I've spoken with did not speak highly of mini splits. I'd say I called 5 and maybe 3 returned my call and so far none have shown with one supposed to come tomorrow. There is so much work in our area getting someone for new work is difficult. Especially for a garage workshop.

The few I spoke with did not speak well of the mini split and prefer the conventional HVAC heat pumps and tell me they are cheaper. I'll keep regular updates here as I get closer. Realistic install date is probably April or May.

Many who don't speak highly of them don't install them. Simple as that.
Very common to use ductless mini splits in computer rooms & small additions.
We use them for a contractor that converts garages to sound studios. In a studio- the system MUST be quiet. Several brands offer a low voltage (40v) indoor section that if you are standing 3 feet away from it, you can't even hear them run. Many like the wireless remote control, dehumidification, simplicity of installation, no ducting, washable filters & low amp draw.
Find a contractor that speaks highly of them.

 

[Falcon67]

My last insulated 20x24x8 cooled well with a 12K window unit. 8' door on the north, south and west sides had full sun exposure. R-13 all around. Ceiling was insulated - broken by 4 each 4x4 sliding doors for attic access. Kraft paper exposed, no drywall. YMMV as they say LOL.

 

[klhitman]

I'm with rick at 500 sf per ton as a rule of thumb but yes there is a lot to take in on sizing the stuff. If you don't mind the noise go with a split system or even to a package unit on the outside. I am a fan of package units as everything right there when u have to work on it.

 

[rickairmedic]

I agree find a contravtor that likes and uses them . I just did a church fellowship hall with 2 ea 2 & 1/2 ton mini splits . Their total was $7000.00 and would have been $12,000.00 to $15,000.00 with a standard split ducted system . I personally dont like package units as they are a royal pain to work on ( no matter what you need to do on one theres 5000 screws to remove ) . I had to do a blower motor on a package unit this winter and would have much prefered to be inside a cool house instead of freezing my tail off out laying in the snow . I also dont like the fact that if an evap coil or heat exchanger goes bad and you cant get it anymore you are replacing the whole system instead of just a component. That I will leave up to preferance though . Your aplication says mini split.

Rick

 

[Jackfre]

First of all, what part of the country do you live in? I can see Rick's "rule of thumb' at 500 sq ft/ ton for Kentucky. I represent Fujitsu in the six New England States and I am perfectly comfortable with 600/ton residentially...and, all the folks talking about running the numbers are correct. With the most efficient mini-split equipment you will get the efficiencies of water source heat pumps without the expense.

 

[rickairmedic]

Hey Jack good to see you in here . Maybe if the OP lists his location you can give him a hookup with a " qualified " installer in his area .


Rick

 

[klhitman]

I agree find a contravtor that likes and uses them . I just did a church fellowship hall with 2 ea 2 & 1/2 ton mini splits . Their total was $7000.00 and would have been $12,000.00 to $15,000.00 with a standard split ducted system . I personally dont like package units as they are a royal pain to work on ( no matter what you need to do on one theres 5000 screws to remove ) . I had to do a blower motor on a package unit this winter and would have much prefered to be inside a cool house instead of freezing my tail off out laying in the snow . I also dont like the fact that if an evap coil or heat exchanger goes bad and you cant get it anymore you are replacing the whole system instead of just a component. That I will leave up to preferance though . Your aplication says mini split.

Rick

it depends on where the package unit is located.in a shop like the OP said he could use a supply and return vent and that's it. but still have the the option to duct to other location like an office if he wanted. and as far as the 5000 screws to remove to work on a package system, you just have to know what you are working on. and i know you will always miss that one screw that you don't see and it pisses you off.

also rick are you a tech, installer, or a contractor? if you ever need some mods on a unit or a curb to set a unit let me know as i work for a local KY h vac modifier company that can meet your needs!

 

[SloppyDogDrool]

I'm in South Carolina.

One installer showed up today. Saw the building studded out with no insulation and recommended a 3 ton conventional unit. I explained the insulation was yet to come and yet he still said a 3 ton unit was the way to go. He had no interest in a mini split. Conventional system, 3 ton, 13 seer roughly $4,500 installed.

Spoke with an installer that supports the mini split systems. He suggested 2.5 ton. Waiting on a rough quote from him.

Spoke with a second installer that supports mini splits. He didn't think a single system would get the job done because of the size of the room. Thought there would be hot and cold spots. He suggested two 15000BTU units on opposite side of room treating the area like two zones. I'm leaning toward the Fujitsu brand. The 30K is 15 seer and the 15K is 20 seer. The efficiency difference and logic he used seemed to make sense.

Keeping my options open, how much know how does it to take to install a mini split system? I can deal with the physical installation and electrical aspect but what about dealing with the gas, bleeding the system or whatever else needs done? Can they be installed on an interior wall and the lines run exposed to an exterior wall?

One of the installers said if I were to buy the systems online there is no warranty. That doesn't seem to make a lot of sense.

Thanks for the input folks!

 

[Xray]

In SC also...

I just installed a 18,000 BTU mini split (all electric) in a well insulated 864 sq ft (main room) garage. I'm very happy with the performance of this system. It's very quiet with the fan on low and not to loud on high. With the fan on high you can feel the heat from the opposite side of the building (24ft). These ductless systems have a motor driven sweep vent/flap to distribute the air up/down and some systems right/left. Takes about 40 minutes to bring the garage from 40* to 70* before cycling off and only drawing 8 amps of power on heat or A/C. Took just about two hours to wire, install and enjoy. If the A/C works as well as the heat... no more sweating while detailing the truck this summer!

 

[ket-tek]

Just a few comments on various things mentioned in some of these posts, based on my own hvac quest at home..

Mini-Splits have been widely used around the world for many years with great success in many other countrys with old building and stacked housing complexes where there is no way to run ducting or place large indoor and outdoor units.

They have been available in the US as well but seem to be gaining popularity in the recent few years.

As mentioned previously any contractor that says they **** just doesn't sell them, or has never installed them/had any experience with them.

I contacted many local places when I started building my home and garage 4 years ago. I got the same responses. They all said they were junk and would not work. I did find a Mr Slim dealer (they sold conventional as well) that said they haven't sold too many, but the ones they have sold/installed they were very impressed with them and offered up references from a couple clients they had put them in if I wanted. The only problem was it was over $12,000 for 2 Mr Slim systems installed. Out of my range.

I purchased two 24,000btu 2-ton Klimaire's directly from the manufacture for about $1200 each (without electric backup heatstrip option). Complete kits included everything for the install, and they were a breeze to DIY. Even thought the units come precharged with freon, I found a hvac tech on the side to come do the start up ~ vacuum it down, leak test, and check the pressures. The linesets had flared threaded fittings, and there could be a leak, as well as you need to get any moisture out of the lineset before startup. I didn't want to open the valve and find out I had a leak by loosing all my freon and have to pay a tech anyway plus buy more freon.

I opened the outdoor unit up and it has a Toshiba compressor in it, same as alot of the other random brands you see on the internet. Warranty? Well I won't really know until I have a problem, but I figured at $2400 for two systems, I could replace both of them 4 more times before I met the $12,000 price tag of two Mr Slims.. Only time will tell, but for now they work great and I'm satisfied.


And for the Manual J, that was something else I asked about calling around (for the house hvac, not the garage), and it blew my mind how many companies I called and they either didn't know what a Manual J was?! or they gave me some line about 'I've been doing this for 20 years I know what I'm doing and exactly what you need without seeing the home or the prints.' They were very offended that I would ask for such a thing, even though it was a service I would gladly pay for.

 

[SloppyDogDrool]

The more I think about this and the high quotes and push back I keep getting the more I think I might try and tackle the install of a single 15,000BTU unit myself or maybe have a tech finish the line hook up as mentioned above. See where the hot and cold spots are then add a second unit as the summer get's hotter.

The area where I would like to install the unit is an interior wall. The lines would have to run 20' to an exterior wall then maybe a couple more feet to allow room to tie into the condenser.

I do question running the lines. In the typical install the lines are outside and I've been lead to believe one line radiates heat and the other cold. I've also been told to keep the runs as short as possible and my 20+' lines will loose a lot of efficiency. Anyone know anything about this?

 

[rickairmedic]

Sloppydog the units themselves are quite simple to install. The only problem with a DIY install is if you dont have the equipment to vascuum the system after its hooked up you will still have to have someone come out and do that and the startup. I will let you in on a little secret . Most Pawn shops I have ever walked into had vacuum pumps and guage sets forsale. Most minisplits come out of the box with the correct amount of refrigerant in them so if you can pull the vacuum yourself and make sure theres no leaks " aaaaand you dont alter the length of the lineset you could do it yourself with a couple of tools from a pawnshop.


Rick

 

[rickairmedic]

The units come from the factory with 15' linesets and it is preferable " especially with a DIY" to keep that length . Yes they can be lengthened but you will then need to add refrigerant to the system.


Rick

 

[rickairmedic]

also rick are you a tech, installer, or a contractor? if you ever need some mods on a unit or a curb to set a unit let me know as i work for a local KY h vac modifier company that can meet your needs!

Hitman My wife and I own an HVAC company so to answer your question all of the above . I do whatever I need to do to keep things running.


Rick

 

[SloppyDogDrool]

All good advice folks. I am pretty sure I can locate a tech for the startup and final connections. Save a few bucks on the electrical and manual labor.

Installation wise the unit is ideally setup on the back wall. However, 30' away on the opposite wall are two large metal garage doors that get full sun all day long and the cook the building. I'll have 1.5" foam board on them but I think that only get's me an R-Value of around 7. I can use the foil backed foam but and pick up a couple r-value points but the foam in that type is a lot heavier and with the size of doors sag is a concern. Two doors, 10x16 and 10x10. Their like radiators in the summer! My point is I'm a bit concerned with that location I would be really cool by the unit and really hot by the doors.

I've gone over budget with my shop and really trying to keep the costs down. However I don't want to screw myself. So all your advice is good. I am curious though. The websites that sell these and the videos on you tube just show them hooking up the lines and pushing a purge valve. Nothings ever been mentioned about vacuum gauges unless you go over a certain length which seems to be either 14' or 35' depending on the manufacturer. Could be that it's "acceptable" but not ideal.

 

[jkeyser14]

All good advice folks. I am pretty sure I can locate a tech for the startup and final connections. Save a few bucks on the electrical and manual labor.

Installation wise the unit is ideally setup on the back wall. However, 30' away on the opposite wall are two large metal garage doors that get full sun all day long and the cook the building. I'll have 1.5" foam board on them but I think that only get's me an R-Value of around 7. I can use the foil backed foam but and pick up a couple r-value points but the foam in that type is a lot heavier and with the size of doors sag is a concern. Two doors, 10x16 and 10x10. Their like radiators in the summer! My point is I'm a bit concerned with that location I would be really cool by the unit and really hot by the doors.

I've gone over budget with my shop and really trying to keep the costs down. However I don't want to screw myself. So all your advice is good. I am curious though. The websites that sell these and the videos on you tube just show them hooking up the lines and pushing a purge valve. Nothings ever been mentioned about vacuum gauges unless you go over a certain length which seems to be either 14' or 35' depending on the manufacturer. Could be that it's "acceptable" but not ideal.

No, you really want to pull a vacuum. If you don't you will get air and moisture in the system. The entire compression & evaporation cycle is based on the thermodynamics of the refrigerant and not air. It will cause poor performance and if enough moisture gets into the system, it could cause catastrophic failure of the compressor.

 

[SloppyDogDrool]

jkeyser14, thanks for that info. Just assures me that is the right way to go.

Xray mentioned he's having no issue with air movement at 24' but I'm a bit concerned since I have such a high ceiling. The ceiling was the draw back with all the installers I spoke with. The advice the mini-split guys gave was to install them around 7' and to not try to agressively cool the buffer of air above 8'. They both agreed trying to cool the extra volume of air would be difficult.

Thanks again to everyone who's replied. I feel like I have enough information to be dangerous.

 

[ket-tek]

Yeah like mentioned, the unit may have the right amount of freon in it for the lineset but the linesets have threaded flared fittings. So pulling a vacuum and purging with nitrogen will eliminate any moisture in the lines and test your connections for leaks.

And I suggest calling the specific manufacture you are looking at and asking about the lineset length requests you may have. Mine came with 16ft lines stock, when I talked to the manufacture they said longer runs are ok but there are height/rise limits. My bonus room above needed much more than 16', so I bought an additional 16' line kit from the manufacture and used flared union couplers to connect the two linesets. So I'm running a 32ft lineset from the ground up stairs approx 24' up, they said it would be fine as the rise limit was more like 35'-40'. I obviously needed more r410 added due to the extra length.

 

[ket-tek]

My garage is 28x38x12 and the blower is about 10.5'-11' up. The manufacture said I might could use thier 18,000k 1.5 ton for the 28x38 but said I may want to try the 24k/2t due to the extra cubic volume of the 12' height. It blows across the 28' no problem.

Attached garage, the unit is in the center of the 38'w on an interior(house side) wall. 6" walls with R19 bats, 2" double sided insulated garage doors, r34 in the ceiling with finished bonus room above with the same size walls and insulation.

 

[Average Joe]

The ceiling height is the one thing I was wondering if any one was going to mention. I was lucky enough to be able to recycle a unit. I have a 100k BTU 90+ furnace, the heat works great but the only problem I had was all my heat went to the ceiling. Before I installed 2 ceiling fans I would be at about 55 on the floor and about 93 at the ceiling. (I forgot to mention I also have a 30'x40'x12' shop, also a pole barn) After installing the 2 ceiling fans I save lots of gas and time heating the place up, now I can be at 55 on the floor and 59 or 60 at the ceiling. All the difference in the world all for about $120.00.

Here is a link to the fans I got, http://www.accessdiscounts.com/sku-78614.html they work great, are quiet, have 5 speeds, and will absolutely get some air moving on high speed. I usually only run them on low speed (1) or the next speed up (2). The only problem withe the fans is they come with an ugly solid state transformer wall control. lol,
I Had to hide them in one of my cabinets. lol

Check out my thread to see my HVAC install, maybe it will give you some Ideas? Hope this helps a little bit,
Joe

 

[BoostAddiction]

My garage is 28x38x12 and the blower is about 10.5'-11' up. The manufacture said I might could use thier 18,000k 1.5 ton for the 28x38 but said I may want to try the 24k/2t due to the extra cubic volume of the 12' height. It blows across the 28' no problem.

Attached garage, the unit is in the center of the 38'w on an interior(house side) wall. 6" walls with R19 bats, 2" double sided insulated garage doors, r34 in the ceiling with finished bonus room above with the same size walls and insulation.

My garage is about the same size- including the 12' height. Similar insulation, and I live in the Mid-Atlantic area.

I have a Daikin minisplit that I recall is just 15K BTU and it does a great job. Not a Model J, but one data point.

Also, mine is now 5 years old, and has worked perfectly, every time I've turned it on. I bought mine over the web, the installer wouldn't warranty it, so I just absorbed the risk. So far, so good.

My unit is mounted opposite the main doors, and the far side of the garage does see somewhat more extreme temps. But overall, the volume seems pretty even for a garage.

 

[Jackfre]

Check out the www.airtec.com line of components. Do not put your unit on a flat pad. Elevate it, either with an Airtec bracket or weld yourself up one. You will keep your unit cleaner and it assists when the unit is in the defrost mode. You will be very pleased with the heating side of things. I've used the brackets for all three of my systems. As well Airtec has a product called slim-duct that will conceal the line set. It end up looking like a downspout. You can paint it out to match the building.

In addition to pumping the system down you MUST run a nitrogen purge. Run that purge up to about 500# and let it sit over-night. It will tell you if you have a leak. Doing so will make it easier to pull a good vacuum. It is imperative that you, number one, get a good flaring tool. your old plumbing tool won't do the job for you. Trust me here...I've screwed this up more than once. Once I got a good Ritchie flaring tool, the first flare I did was walk over to my tool box and throw my old plumbing flaring tool in the garbage. I didn't want to be tempted to give it away and pass its legacy along. The number one mistake in mini-split installs is improper flaring tools leading to refrigerant leaks.

My suggestion is to mount the equipment, run the line set. If you don't have the proper flaring tool have the tech connect things, purge, evacuate and charge the system. Do it, get it done, once...do it right and you will be very pleased with these unit(s). Air flow on a 15RLS will not be a problem. It will handle 24 ' easily. BTW, the 15RLQ has been replaced by the 15RLS for 2011. Seer went up to 21.

 

[Jackfre]

Hey Jack good to see you in here . Maybe if the OP lists his location you can give him a hookup with a " qualified " installer in his area .


Rick

thanks, Rick! Go to www.fujitsugeneral.com and the dealer locator will give you a number of options. I don't know the contractors there so, just follow the yellow brick road

 

[SloppyDogDrool]

I have another consideration which is power consumption. My pole barn is on a separate meter which in my area when you have a second meter on the same property you have to pay commercial rate. So my power costs are higher to start with. Efficiency is a bit more of a concern with higher power costs.

With all your suggestions I've boiled it down to either a single 15 SEER 36K BTU Mitsubishi or two 21 to 22 SEER 15K Fujitsu or Mitsubishi units.

My thought is that one 15K unit may suffice in the moderate spring and fall time and in full summer or winter both units will be required. The cost difference is roughly $700 between a single 36 or a pair of 15.

At startup both unit could run and then one could shut off. When I'm not in the shop one unit could stay running to maintain a moderate temp like 82 degrees and help control humidity.

Just some thoughts. Whats your feedback?

 

[Jackfre]

Carefully check the units specs. The 36k Mitsu hp will produce 31.4kbtu in cooling. Fujitsu's 30kbtu unit will make 32,400 max.

AC typically can be expensive because the central equipment is mostly single stage. It operates the way you drive at the drag strip. Mini's are variable speed. They modulate based upon conditions. They operate the way you actually drive, a little gas when going slow, when you need to get on the hwy, you boot it and go. Much more efficient than the drag strip. A good part of their efficiency comes from having fewer starts and stops. With conventional equipment the power draw on start-up is significant. With the mini-split they open the expansion valves prior to making the compressor, so the pressure is equalized and the compressor doesn't have to overcome the head pressure of the system on start-up. These thing de-hu really well too!

I think you might consider a single 24 to get away with a single unit. 18-19 seer. Check me on that. Don't recall exactly. I own two of the 15's you reference and they are great, but keep in mind, they are all variable speed so all of them will modulate to the load. If that 24 is to small then you will have defined the hot spots and can get a better sense of your need. Now, I'm a Yankee, so I don't have to deal with the loads you do, but I've found that these things consistently exceed your expectations...given a good install

 

[ket-tek]

As far as power usage these things run on such low power draw I don't think it's gonna have the major impact your imagining. Especially if you looking at the very high seer models. Mine run on a single breaker that feeds straight to the outdoor unit, then the control connections from the outdoor feed to the blower inside also provide the power. So no need to run power from the panel to both locations.

My conventional 1.5 ton pump for the downstairs of my house takes both 15amp and 40amp breakers and the 2 ton upstairs takes 25amp and 60amp. Compared to the 2 ton mini's I have only take a single 20amp feed.

Mini's are variable speed.

Great info to point out, which helps with power usage as well since it only runs as hard as necessary for the conditions.

 

[ket-tek]

The ceiling height is the one thing I was wondering if any one was going to mention.

The only problem withe the fans is they come with an ugly solid state transformer wall control. lol,
I Had to hide them in one of my cabinets. lol

Yeah, fans can make a big difference in helping eliminate stale/dead zones and get the heat off the ceiling. I have a white one similar to the one you posted, those brushed nickel are sweet looking! And cheaper than the white one was I have!

Ah, come on those controllers don't look too bad. The gold numbers I guess make it look a little 80's tho..

 

[SloppyDogDrool]

ket-tek, I keep hearing of situations where people are getting away with smaller units than they should. Your logic of a 24K to start makes sense. Helps my budget and would probably hold me until the hottest days of July and August. The building hits over 100 degrees on those days with the doors closed and no air moving.

makes sense also to see where the unit takes me. Add another 24k or a 15K if I just need a little more.

I don't want to be cold in the winter or overly hot in the summer. I don't intend to bring the room down to 68 degrees but 75 would be nice. Much over that and it starts to get kinda warm to be working.

Off topic. Someone mentioned the window units. I have one in my 360sqft fun building. It's rated for 700 sqft. It's great at cooling but when it gets cold, not so great. The air is warm but not hot. Takes about 4 hours to get the room to the point you can be in a T-Shirt playing pool. Cooling is much better except it can't move enough air to keep the whole room consistent. The window unit was a great fix for my situation. Less than $300 from Lowe's during the tax free holiday and on clearance.

 

[SloppyDogDrool]

Fans, I don't really have a goo location for a fan. bay 1 has a 4 post lift with a side opening garage door opener. Bay 2 is going to have an air cleaner for my wood working.

I guess I should have mentioned that somewhere early on. My space will be used for car maintenance, wood working and all my other hobbies. More hobbies than time.

 

[Jackfre]

With your woodworking, do you have dust collection? I love woodwork, but hate sawdust. As such I have a JDS ambient air filtration, a Delta primary DC and assorted shop vacs. If you do not have similar equipment you are going to have to get used to cleaning the evap coil. To attain the efficiencies we are hitting, the coils are quite fine. You want them clean.

For fan alternatives, look at wwwtjernlund.com at the Airshare and Xchanger products. They may be of use to you.

 

[SloppyDogDrool]

Jackfre, I do have a 3.5 HP cyclone tempest from Penn State Industries. In process of installation. I will have an overhead filter as well after I get rolling and see how bad a dust issue I have. I have purchased tools with high collection capabilities. Because I use the space for other things I don't want a ton of dust everywhere. I'll probably post a thread somewhere else with the build. It's been a 5 year progressive build.